Backed by remarkable improvement in the light emitting efficiency, the market of light-emitting diodes (LEDs) has been rapidly expanding for backlights of liquid crystal displays (LCDs), vehicle headlights, spotlights and general lighting applications that are characterized by low power consumption, long service life and excellent design.
The emission color of an LED is limited because its emission spectrum is dependent on the semiconductor material for forming an LED chip. Therefore, in order to obtain white light from an LCD backlight or general lighting apparatus by using an LED, it is necessary that a phosphor suitable for an LED chip is arranged on the LED chip to convert an emission wavelength. Specifically, there have been proposed, for example, a method of arranging a yellow phosphor on a blue light-emitting LED chip, a method of arranging red and green phosphors on a blue light-emitting LED chip, and a method of arranging red, green and blue phosphors on a UV-emitting LED chip. Thereamong, from the standpoints of the light emitting efficiency and the cost of the LED chips, a method of arranging a yellow phosphor on a blue LED and a method of arranging red and green phosphors on a blue LED are most widely employed at present.
As a concrete method for arranging a phosphor on an LED chip, a method in which a phosphor is dispersed in a sealing resin of an LED chip has been proposed (see, for example, Patent Documents 1 and 2). However, when a phosphor is dispersed in a liquid sealing resin, it may not be able to supply the resulting liquid to each LED chip at a constant amount due to poor dispersion caused by precipitation of the phosphor having a large specific gravity. This leads to thickness variation of the sealing resin as well as the concentration of the phosphor, which causes color differences from one LED chip to another in some cases.
As a method of inhibiting such precipitation of a phosphor in a sealing resin, for example, a method of adding a thixotropic agent (see, for example, Patent Document 3) and a method of adding silicone fine particles (see, for example, Patent Document 4) have been proposed; however, when a phosphor is dispersed at a high concentration, it is difficult to sufficiently inhibit the precipitation of the phosphor.
In view of this, there has been proposed a method of using a resin in which a phosphor is uniformly distributed at a high concentration after molding it into a sheet form (see, for example, Patent Document 5). In this method, by molding a resin containing a phosphor at a high concentration into a sheet form in advance, uniform sheet thickness and uniform concentration distribution of the phosphor as well as light resistance can be attained; therefore, color variations of LEDs can be inhibited when the sheet is pasted to an LED package or a light-emitting element.